1. Field of the Invention
The present invention relates to a measuring technique for measuring a magnetic field vector, particularly useful for mapping the distribution of electric currents in a sample in order to locate defects.
2. Description of the Background Art
Devices for measuring the distribution of a magnetic field are known and widely used. Such a device typically comprises either a single sensor, or probe, mounted for movement along the surface of a sample to be inspected, or a stationary arrangement comprising an array of such probes.
FIG. 1 illustrates the main principles of operation of the conventional magnetic sensor such as, for example, a Hall sensor. Hall sensors are based on the known Hall effect according to which a magnetic field applied to a semiconductor, along which an electric current flows, produces a voltage across the semiconductor in a direction perpendicular to the magnetic field and the current directions. A Hall sensor, generally designated 1, typically has an active element 2 and two pairs of ohmic contacts 2a-2b and 3a-3b. An electric current I flows between the contacts 2a-2b aligned in the direction x. This current I, the magnitude and direction of which are known from a calibration stage, in the presence of a perpendicular magnetic field, generates a respective Hall voltage Vy in the contacts 3a and 3b aligned in a transverse direction y. As known, a Hall sensor is sensitive to that component of the magnetic field which is perpendicular to its surface. More specifically, the Hall voltage Vy is responsive to the current flow I and to the strength of a magnetic field provided within the vicinity of the sensor 1 and directed perpendicular to the surface of the active element 2. Thus, a component Bz of the magnetic field B is measured. All these particulars are well known per se and, therefore, need not be described in more detail.
It is appreciated that in order to determine the gradient of the magnetic field B, within the vicinity of a conductive material, and thereby the actual magnitude and direction of the electric currents inside the conductive material, the magnetic field at different locations relative to the conductor should be determined. Although this information may be obtained by moving a single Hall sensor across the conductor, stationary arrangements of linear Hall sensor arrays have been developed by producing a row of such sensors aligned in a straight line. FIG. 2 illustrates the geometrical arrangement of a device of this kind, generally designated 4, comprising a pair of Hall sensors 6 and 8. The device 4 is located within a magnetic field B which is either externally applied magnetic field or induced by an electric current. The sensors 6 and 8 are aligned in a line extending in a direction x. The magnetic field components Bz1 and Bz2 are measured independently by each of the sensors 6 and 8 by the way of direct measurement of Vy1 and Vy2. Hence, the gradient of the magnetic field component Bz along the direction x can be calculated as:                                           ∂                          B              z                                            ∂            x                          =                                            B                              z                1                                      -                          B                              z                2                                              L                                    (        1        )            
wherein L is the known distance between the sensors 6 and 8.
A device of this kind is disclosed, for example, in the article xe2x80x9cAutomatic Devices for the Measurement of Flux Density Gradientsxe2x80x9d, H. W. Weber et al., Cryogenics, 1976. The device comprises eleven field probes made of InSb and mounted on a narrow gap between the two halves of a sample. Output voltages of the probes are recorded simultaneously in order to provide a complete description of the magnetic field distribution at eleven positions along the sample radius.
Turning back to FIG. 2, it is understood that the smaller the dimensions of each of the sensor and the distance L between the sensors 6 and 8, the higher the resolution of the device 4. It is often the case that the local value of a magnetic field should be measured rather than global, for example for inspecting high temperature superconductors.
A device of this kind is disclosed, for example, in the article xe2x80x9cLocal Magnetization Measurements in High Temperature Superconductorsxe2x80x9d, D. Majer et al. The device presents magnetization measurements in which a magnetic response to an externally applied field is investigated. The main purpose of the device is to provide local values of the magnetic induction B inside the sample. To this end, the device comprises arrays of substantially small Hall sensors each extending in a plane parallel to the sensors"" surfaces and formed in a two dimensional electron gas (2DEG) material. These sensors have the advantage of a linear response to magnetic field, weak temperature dependence and high sensitivity. The advantage of 2DEG material is the ability to make several Hall sensors on the same device for measuring the magnetic induction across and outside the sample and giving a detailed local structure of the magnetic profile without limitation from sample""s dimensions.
It is thus evident that according to the conventional approach as described above, the array of spaced parallel Hall sensors extends in the plane parallel to the sensors"" surfaces. Each of the sensors measures the component Bz of the magnetic field induction associated with respective location (xi;yj) on the sample. If the array extends in the direction x, as exemplified in FIG. 2, the spatial distribution of the perpendicular component Bz(x) is mapped.
However, the distribution of the other two components Bx(x;y) and By(x;y) of the magnetic field B, i.e. components parallel to the sensors"" surfaces, cannot be measured by the conventional device employing magnetic sensors of any known kind. This information is very important, for example, for mapping electric currents inside a conductive material in order to make a useful diagnostic tool for finding features like cracks in the conductive material.
A device for measuring magnetic properties has been developed and disclosed in the article xe2x80x9cThree-Axis Cryogenic Hall Sensorxe2x80x9d, J. Kvitkovic et al., Journal of Magnetism and Magnetic Materials, 1996. The device comprises three independent Hall sensors glued to a supporting ceramics and located at the corner edge thereof for detecting the spatial field profile within a small cube. The Hall sensors are arranged in such a manner that centers of their active areas are placed in three mutually perpendicular planes. The sensors are supplied by a single constant current source. A sample to be inspected is placed in an external magnetic field region. It is appreciated that such an arrangement of the device enables the magnetic field components to be measured along three directions x, y and z. However, the manufacturing and operation of the device are complicated requiring gluing processes and displacement of the device in order to obtain the map of a magnetic field vector.
It is often the case that a conductive structure has to be inspected without destroying the usefulness thereof. In other words, the contact to a conductive structure so as to directly connect it to a power source may be undesirable and/or impossible. Indeed, it turns out to be very difficult to place reliable electrical contacts on the surface of many conductive structures and in many cases a structure to be inspected in not accessible for attaching contacts. One of the conventional diagnostic techniques usually employed for inspecting such a conductive structure, the so-called xe2x80x98eddy current techniquexe2x80x99, is based on the finding that an electric current flowing inside the structure is induced by an external alternating magnetic field. The standard way of measuring the magnetic field generated by eddy currents is based on the same process that generated the eddy currents, i.e. magnetic induction. A small coil, or array of small coils, is placed over the conductive structure and used for monitoring changes in the magnetic field patterns associated with the eddy currents.
However, the use of the magnetic induction method requires that the magnetic fields change in time. Additionally, such coils are sensitive to the rate of change, i.e. frequency, of the magnetic fields to be measured. When eddy currents are generated in a conductor by a changing magnetic field, the depth to which these currents are produced depends upon the frequency of the magnetic field. This is one of the reasons for the limited success of the conventional magnetic induction technique for structures made of good conductors such as, for example, aluminum used in aircraft structures, wherein eddy currents are produced only in a very thin layer near the surface of the structure. This penetration depth for alternating magnetic field greatly limits the applicability of the conventional eddy current method.
It is thus a major object of the present invention to provide a novel device for measuring a magnetic field distribution, particularly such a device for determining the complete 3D profile of a magnetic field vector.
It is a further object of the present invention to provide such a device that allows simultaneous measurements of the components of the magnetic field which are parallel and perpendicular to the surface of the device.
It is a still further object of the present invention to provide such a device that enables inspection of a conductive structure for detecting defects, if any, in a non-contact manner.
There is thus provided according to the most general aspect of the present invention a measuring device for determining the spatial distribution of a magnetic field vector, comprising at least a pair of sensor elements each for measuring a component of the magnetic field vector, the sensor elements being aligned in a parallel, spaced-apart relationship along an axis parallel to the measured components.
The term xe2x80x9ccomponentxe2x80x99 as used herein should be understood to mean a projection of a vector on one axis of a cartesian coordinate system (i.e. along the xe2x80x9cxxe2x80x9d, xe2x80x9cyxe2x80x9d or xe2x80x9czxe2x80x9d axis).
The sensor elements are spaced from each other at a known small distance. The sensor elements are supported for a movement within a plane perpendicular to the axis of the alignment of the sensor elements. The magnetic field spatial distribution is determined by utilizing Maxwell""s Laws for estimating the other two components of the magnetic field vector.
The device may comprise an array of a plurality of pairs of the sensor elements. The array may be a linear array or a two dimensional array. The array extends in a direction perpendicular to the axis of alignment of the sensor elements of each pair. The array may be supported for a movement within a plane perpendicular to the axis of the alignment of the sensor elements of each pair.
The magnetic field whose spatial distribution is to be determined is a field generated by an electric current passing through a conductive structure, wherein the device is accommodated within a vicinity of the conductive structure. The electric current may be a transport current generated by a power source directly coupled to the structure. In this case, the device may be used for quantitative determination of magnitude and direction of the transport current.
The electric current passing through the conductive structure may also be an xe2x80x98eddy currentxe2x80x99 induced by an external, alternating, magnetic field applied to the conductive structure. In this case, the device may be used to evaluate a profile of the electric current inside the structure by comparing it to an associated reference data.
Each of the sensor elements may be a Hall sensor, coil, magneto-optic detector, etc. In case of Hall sensors, they may be formed in at least one pair of parallel, spaced-apart layers of semi-conductive materials, e.g. two-dimensional electron gases.
According to another aspect of the present invention, there is provided a method for determining the spatial distribution of a magnetic field vector within the vicinity of a conductive structure, comprising the steps of:
(a) bringing at least one pair of spaced parallel magnetic sensors each for measuring a component of a magnetic field vector in a vicinity of the conductive structure and aligning the sensors along a z axis parallel to the measured components;
(b) measuring with said at least one pair of magnetic sensors a gradient of the magnetic field component, Bz(z); and
(c) determining at least one of the magnetic field components, Bx(x;y) or By(x;y) oriented perpendicular to one another in a plane perpendicular to said z axis.
The method may also comprise the step of calibration for determining the space between the sensors of said at least one pair of sensors. In the event that an external magnetic field is applied for inducing electric currents in the conductive structure, magnitude and direction of the external magnetic field may also be determined at the calibration step. The components Bx(x;y) and By(x;y) are determined by utilizing Maxwell""s Laws.
The method may also comprise the step of estimating a profile of the electric current passing through the conductive structure either by utilizing Ampere""s Law, or by comparing the determined distribution of the magnetic field vector within the vicinity of the conductive structure to a corresponding associated reference data.
According to yet another aspect of the present invention there is provided a method of fabricating a miniature measuring device for determining the spatial distribution of a magnetic field vector, comprising the steps of:
(i) providing a structure consisting of at least two parallel, spaced-apart layers each formed of a conductive material;
(ii) forming at least a first pattern of regions on one of said at least two layers so as to define at least one magnetic sensor;
(iii) forming at least a second pattern of regions on another of said at least two layers so as to define the at least one other magnetic sensor; and
(iv) fabricating said ohmic contacts within the formed patterns.
Step (i) preferably includes growth of the structure on an undoped semi-insulating substrate. The growth of the structure may be achieved by either chemical or physical deposition techniques such as, for example, molecular beam epitaxy or sputtering. The ohmic contacts may be formed by lithography and deposition processes as generally known.
More specifically, the present invention is used with Hall sensors and is therefore described below with respect to this application.